This invention relates generally to improvements in a compressor of the type used primarily for air conditioning applications. More specifically, this invention relates to an improved compressor, preferably of the progressive cavity type, designed for improved efficiency particularly at part load operating conditions.
Rene Moineau did his Ph.D. research and thesis on the progressive cavity pumping principle. (Devices employing Moineau's geometry are known, variously, as “Moineau”, “progressive cavity”, or “progressing cavity” devices). The first of his ten or more U.S. patents, U.S. Pat. No. 1,892,217, was issued in 1932. This patent mentions varying-pitch and is intended to include applicability to compressible fluids. To date, progressive cavity pumps have been used mainly to pump viscous liquids, such as petroleum, or to handle liquids containing solid material, such as drilling fluids.
A few progressive cavity compressor patents have been issued, e.g., Fujiwara, U.S. Pat. No. 4,802,827. In addition, several progressive cavity pump patents describe or claim applicability to compressible fluids, including liquid-gas mixtures. For one example, see Varadan, U.S. Pat. No. 6,093,004.
To date the progressive cavity principle has seen little or no use in any compressor application. In vapor-cycle systems of 3 to 10 ton capacity, the scroll compressor and the piston compressor are dominant. Both types are mass-produced at relatively low cost, but generally have not included features that promote good off-design energy efficiency.
The present invention is aimed to compete against the well established piston and scroll compressors in air conditioning applications, by means of superior energy efficiency, especially at part load conditions on cooler days. The invention can also be usefully applied to other compressor applications for which the required compression ratio varies, and for which off-design energy efficiency is important.
Some vapor-cycle compressors used for air conditioning are designed for a fixed compression ratio that matches a maximum outside ambient air temperature. The compressor is run in an inefficient off-design mode on days when the ambient temperature is below this maximum. To promote off-design energy efficiency, the present invention operates efficiently over a range of compression ratios, corresponding to a range of outside ambient air temperatures.
The U.S. Department of Energy (DOE) has issued regulations calling for improvement of the energy efficiency of vapor-cycle air conditioning and refrigeration systems. In promulgating these regulations, DOE has established “SEER” (Seasonal Energy Efficiency Ratio) ratings which promote off-design energy efficiency, including efficient operation at various outside ambient air temperatures. The present invention responds to these SEER regulations.
Moineau Configuration in General
A fairly complex mathematical theory defines a family of rotor and stator shapes that result in the formation and progression of sealed cavities through a Moineau pump or compressor. In this family, the rotor and stator both have lobes, and the number of stator lobes is always one greater than the number of rotor lobes. The simplest possible case is one rotor lobe and two stator lobes. In this simplest case each rotor cross section is circular (diameter Dr), and each stator cross section consists of two semicircles (of diameter Ds), separated by a rectangle of dimension Ds×H as shown in FIG. 2. Ds equals Dr plus 2δ, where δ is the small clearance between the rotor and the stator. For a more complex configuration having more than two stator lobes, each stator cross section is defined by a number of semicircles arranged symmetrically about the stator axis and corresponding in number with the number of stator lobes, and separated by a more complex geometric figure.
A Moineau rotor has two motions relative to the stator: a “planetary” rotation about the symmetry axis of the stator, and a “spin” rotation about its own axis. These rotations are in opposite directions. The symmetry axis of the stator and the rotation axis of the rotor are parallel to each other, and are separated by a constant distance, which is the design parameter known as axes separation, or SEP. This separation is enforced by a pair of crank arms or something similar, outside the fluid region, which rotate around the symmetry axis of the stator, and support the two ends of the eccentrically mounted rotor.
Moineau Pumps
No valves are needed in a Moineau pump because the pumped fluid is incompressible. The standard Moineau pump has no special requirements as to outlet-end geometry. While fluid is being expelled from a cavity that is open to the pump discharge, the pressure in the cavity will automatically assume the discharge pressure downstream of the pump, plus the small pressure drop through the discharge port(s).
Valveless Varying-Pitch Moineau Compressor
One pre-existing concept is to create a progressive cavity compressor by altering a progressive cavity pump so that the volume of each cavity decreases as the cavity moves through the working section of the machine. This can be done in any of several ways, for example by means of a rotor and stator that are (a) varying-pitch; (b) cone-shaped; or (c) made up of parallel curves—all as discussed in more detail herein. The net result is a volumetric compression ratio determined by the geometry, and a corresponding pressure ratio determined (ideally) by the compression ratio and the gas laws for the working fluid. This type of compressor can operate efficiently without valves if its main use is at or near the inlet and outlet pressures for which it was designed. At off-design conditions, there will be a pressure mismatch between the outlet plenum and a cavity about to be vented. The result, in the valveless compressor, is a loss of efficiency from the sudden inflow or outflow of the working gas to or from a newly vented cavity.